Light-permeable resins are widely used as materials for forming film-, plate-, and lens-like optical members for use in various optical devices (e.g., films, substrates, and prism sheets for use in liquid crystal display devices; lenses in lens systems for signal reading of optical disk devices; Fresnel lenses and lenticular lenses for projection screens). Such resins are generally called “optical resins” or “optical polymers”.
Birefringence is one of important optical characteristics that should be taken into consideration when optical resins are used to form optical members. Particularly, in the above-mentioned applications (liquid crystal display devices, optical disk devices, projection screens, etc.), the presence of a birefringent film or lens in an optical path adversely affects image quality or signal reading performance, and therefore the use of an optical member made of an optical resin whose birefringence is as low as possible is desired. Needless to say, camera lenses, eyeglass lenses, and the like also preferably have low birefringence.
Birefringence of an optical polymer includes “orientation birefringence” mainly caused by the orientation of main chains of the polymer and “photoelastic birefringence” caused by stress. Orientation birefringence and photoelastic birefringence have their respective signs that depend on the chemical structure of a polymer, and are properties intrinsic to individual polymers.
Orientation birefringence generally develops when main chains of a chain-like polymer (polymer chains) are oriented, and this orientation of main chains occurs in a process involving the flowing of a material, such as an extrusion-molding or stretching process during production of a polymer film or an injection molding process frequently used for production of optical members with various shapes, and is fixed and remains in an optical member. Hereinafter, the phrase “orientation birefringence is positive” means that a refractive index is large in a direction parallel to the orientation direction of polymer chains, and the phrase “orientation birefringence is negative” means that a refractive index is large in a direction orthogonal to the orientation direction of polymer chains.
On the other hand, photoelastic birefringence is caused by elastic deformation (distortion) of a polymer. In the case of an optical member using a polymer, elastic deformation (distortion) occurs and remains in its material due to, for example, volume contraction caused by cooling from a temperature around the glass transition temperature of the polymer to a temperature lower than that, which becomes the cause of photoelastic birefringence. Further, the material is elastically deformed also by, for example, external force exerted on the optical member fixed to a device used at ordinary temperature (glass transition temperature or lower), which causes photoelastic birefringence. As shown by the following formula, a photoelastic constant is defined as a coefficient γ of Δσ when a birefringence difference Δn is caused by a stress difference Δσ.Δn=γΔσ
Hereinafter, the phrase “photoelastic birefringence is positive” means that a refractive index is large in a direction parallel to a direction in which tensile stress is applied (direction in which polymer chains are oriented), and the phrase “photoelastic birefringence is negative” means that a refractive index is large in a direction orthogonal to a direction in which tensile stress is applied.
There are various reports about suppression of the above-described birefringence.
For example, Patent Document 1 discloses a non-birefringent optical resin material obtained by blending two polymer resins that are opposite in the sign of orientation birefringence to each other and are completely miscible. However, it is difficult to uniformly mix the two polymer resins described in Patent Document 1 to obtain a practical polymer resin that uniformly exhibits low orientation birefringence as a whole, and aggregates of the polymer resins may cause foreign matter defects. Further, the polymer resins blended are different in their intrinsic refractive index, and therefore light scattering occurs due to non-uniformity of refractive index, which makes it impossible to obtain an optical material excellent in transparency. Although there is no description about photoelastic birefringence, it is conceivable that a polymer composition of Example will have significantly high photoelastic birefringence. Further, the mechanical strength, especially impact resistance, of the optical resin material is not always sufficient, which causes a problem such as cracking when the optical resin material is practically used.
Patent Document 2 discloses a method for obtaining a non-birefringent optical resin material by adding, to a matrix composed of a transparent polymer resin, a low-molecular material whose orientation birefringence tends to cancel out the orientation birefringence of the polymer resin material. The low-molecular material has a molecular weight of 5,000 or less, and a resulting molded body has excellent transparency. However, there is no description about improvement in photoelastic birefringence or mechanical strength. Further, there is a case where heat resistance is reduced.
Patent Document 3 discloses a method for obtaining an optical resin material having low orientation birefringence by adding, to a transparent polymer resin, a birefringent fine inorganic substance that is oriented in the same direction as the linked chains of the polymer resin as the polymer resin is oriented by the application of external force. Orientation birefringence can be suppressed also by this method, but there is no description about improvement in photoelastic birefringence or mechanical strength.
Patent Document 4 discloses a method for obtaining a non-birefringent optical resin material having low orientation birefringence and low photoelastic birefringence, in which an optical material having a multicomponent system of three or more components including a binary or higher copolymerization system is obtained by selecting the combination and constituent ratio (compositional ratio) of components of the multicomponent system so that both the orientation birefringence and photoelastic birefringence of the optical material are cancelled out at the same time. This method makes it possible to extremely reduce both orientation birefringence and photoelastic birefringence at the same time, which could not heretofore be achieved. However, the composition of the optical resin material is limited to some extent to make it possible to cancel out both orientation birefringence and photoelastic birefringence at the same time, and therefore the glass transition temperature of the optical resin material is as low as less than 100° C. and there is a problem such as a reduction in mechanical strength. Further, there may be a problem that polymer decomposition occurs during molding performed under such conditions that the optical resin material is retained at high temperature, such as melt-extrusion molding for forming film.
The optical resin materials of Patent Documents 1 to 4 improve the birefringence but has problems in practical use as described above. For the purpose of providing a practical optical resin material, improvements of the mechanical strength and heat resistance of the optical resin material have been studied.
For example, Patent Document 5 discloses a method for obtaining a resin composition, which has high heat resistance and exhibits excellent mechanical strength, especially bending resistance, when formed into a film, and an optical film. The resin composition is obtained by using, in combination, an acrylic resin having a glass transition temperature of 120° C. or higher and a graft copolymer obtained by graft copolymerization of a vinyl-based polymerizable monomer to an acrylic rubber-like polymer (“core-shell”-type impact resistance improver, hereinafter also referred to as “core-shell polymer”). The graft copolymer is added for the improvement of the mechanical strength. However, an improvement of birefringence such as orientation birefringence or photoelastic birefringence has not been studied.
Patent Document 6 discloses an optical film obtained by molding a resin composition containing an acrylic resin (A) and an acrylic rubber (B). The acrylic resin (A) is a heat-resistant acrylic resin (A-1) containing a repeating unit derived from a methacrylate monomer, a repeating unit derived from a vinyl aromatic monomer, a repeating unit derived from a methacrylate monomer having an aromatic group, and a cyclic acid anhydride repeating unit. This document states that the optical film has high heat resistance and excellent trimming property and has excellent optical characteristics even when stretched. The graft copolymer (core-shell polymer) that is the acrylic rubber (B) is added to improve mechanical strength while maintaining transparency such as haze. It should be noted that the orientation birefringences of the optical films of Examples to which the acrylic rubber (B) is added are higher than those of optical films of Comparative Examples using only the acrylic resin (A), and the photoelastic constants (photoelastic birefringences) of the optical films of Examples are equal to those of the optical films of Comparative Examples using only the acrylic resin (A).